Method for treating a semiconductor

ABSTRACT

Methods for treating a semiconductor material, and for making devices containing a semiconducting material, are presented. One embodiment is a method for treating a semiconductor material that includes a chalcogenide. The method comprises contacting at least a portion of the semiconductor material with a chemical agent. The chemical agent comprises a solvent, and an iodophor dissolved in the solvent.

RELATED APPLICATIONS

This application claims priority from U.S. nonprovisional applicationSer. No. 13/682,876, filed Nov. 21, 2012, pending and bearing the title“METHODS FOR TREATING A SEMICONDUCTOR.” The above-mentionednonprovisional application is incorporated herein by reference in itsentirety.

BACKGROUND

The invention generally relates to methods for treating a semiconductormaterial. More particularly, the invention relates to methods fortreating a semiconductor layer used in thin-film photovoltaic devices.

Thin film photovoltaic devices typically include a plurality ofsemiconductor layers disposed on a transparent substrate, wherein afirst semiconductor layer serves as a window layer and a secondsemiconductor layer serves as an absorber layer. The window layer allowsthe penetration of solar radiation to the absorber layer, where theoptical energy is converted to usable electrical energy. In certainconfigurations, thin film photovoltaic devices may further include anadditional semiconductor layer interposed between the window layer andthe absorber layer that may function as an intrinsic layer. Cadmiumtelluride/cadmium sulfide (CdTe/CdS) heterojunction-based photovoltaicdevices are one such example of thin film solar cells, where a cadmiumtelluride (CdTe)-based semiconductor layer may function as an intrinsiclayer or an absorber layer.

However, CdTe-based photovoltaic devices typically demonstraterelatively low power conversion efficiencies, which may be attributed toa relatively low open circuit voltage (Voc) in relation to the band gapof the material which is due, in part, to the low effective carrierconcentration and short minority carrier lifetime in CdTe. Effectivecarrier concentration of CdTe may be improved by doping with p-typedopants.

Further issues with improving the device efficiency of CdTe solar cellsinclude the high work function of CdTe and high back-contact resistanceat the interface between CdTe and metal-based back contact layer. Theback-contact resistance may be improved by increasing the carrierconcentration at the back interface. For example, for a p-type CdTematerial, increasing the carrier concentration amounts to increasing thep-type carriers in the CdTe material to form an “ohmic contact layer” onthe backside of the CdTe layer, which is in contact with the backcontact layer.

Typical methods employed to form the ohmic layers or for doping theabsorber layer include etching of the CdTe layers and incorporation ofcopper into back-end of line processing of the absorber layer. However,it may be difficult to control the amount of copper incorporated in thebulk and in the back interface, using a typical CdTe processing method.Further, photovoltaic devices manufactured using the typical methods mayinclude a high copper content at the back-interface, which may adverselyaffect the long-term stability. Furthermore, etching of the CdTe layerusing conventional etching agents may lead to removal of CdTe materialfrom the surface, and selective etching of grain boundaries, resultingin increased defects.

Thus, there is a need for improved methods of processing semiconductorlayers. Further, there is a need for improved photovoltaic deviceconfigurations including the semiconductor layers.

BRIEF DESCRIPTION OF THE INVENTION

Embodiments of the present invention are provided to meet these andother needs. One embodiment is a method for treating a semiconductormaterial that includes a chalcogenide. The method comprises contactingat least a portion of the semiconductor material with a chemical agent.The chemical agent comprises a solvent, and an iodophor dissolved in thesolvent.

Another embodiment is a method for treating a semiconductor material,comprising (a) contacting at least a portion of the semiconductormaterial with a passivating agent, wherein the semiconductor materialcomprises a chalcogenide; (b) forming a first region in thesemiconductor material by introducing a dopant into the semiconductormaterial; (c) forming a chalcogen-rich region; and (d) forming a secondregion in the semiconductor material, the second region comprising adopant, wherein an average atomic concentration of the dopant in thesecond region is greater than an average atomic concentration of thedopant in the first region. Step (c) comprises contacting at least aportion of the semiconductor material with a chemical agent, wherein thechemical agent comprises a solvent and an iodophor dissolved in thesolvent.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, whereinthe FIG. 1 is a schematic cross section of a semiconductor material usedin embodiments of the present invention.

DETAILED DESCRIPTION

Some of the embodiments of the invention include methods for treating asemiconductor layer. More particularly, the invention relates to methodsfor treating a semiconductor layer used in thin-film photovoltaicdevices.

As noted earlier, the conventional methods of making photovoltaicdevices typically include etching and introduction of copper at theback-end line processing. However, it may be difficult to control theamount of copper incorporated in the bulk and in the back interface,using a typical CdTe processing method. Further, photovoltaic devicesmanufactured using the typical methods may include a high copper contentat the back-interface, which may adversely affect the long-termstability. Furthermore, etching may lead to grain boundary modificationand electrical shunting. Embodiments of the invention described hereinaddress the noted shortcomings of the state of the art. Embodiments ofthe present invention advantageously provide for efficient and stablephotovoltaic devices, and methods of making these.

In the following specification and the claims, the singular forms “a”,“an” and “the” include plural referents unless the context clearlydictates otherwise. As used herein, the term “or” is not meant to beexclusive and refers to at least one of the referenced components beingpresent and includes instances in which a combination of the referencedcomponents may be present, unless the context clearly dictatesotherwise.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about”, and “substantially” is not to be limited tothe precise value specified. In some instances, the approximatinglanguage may correspond to the precision of an instrument for measuringthe value. Here and throughout the specification and claims, rangelimitations may be combined and/or interchanged, such ranges areidentified and include all the sub-ranges contained therein unlesscontext or language indicates otherwise.

The terms “transparent region” and “transparent layer” as used herein,refer to a region or a layer that allows an average transmission of atleast 70% of incident electromagnetic radiation having a wavelength in arange from about 300 nm to about 850 nm. As used herein, the term“disposed on” refers to layers disposed directly in contact with eachother or indirectly by having intervening layers therebetween, unlessotherwise specifically indicated.

The terms “contacting” or “contacted” as used herein means that at leasta portion of the semiconductor layer is exposed to, such as, in directphysical contact with a suitable contacting material, such as, forexample, a passivating agent, a chemical agent, a dopant source, acontacting composition, a metal salt, or combinations thereof. In someembodiments, a surface of the semiconductor layer may be contacted withthe suitable contacting material, for example using a surface treatmenttechnique. In some other embodiments, a substantial portion of thesemiconductor layer may be contacting with a suitable contactingmaterial, for example, using an immersion treatment.

In the specification, drawings, and in the claims, the embodimentsrelated to the methods are not limited by a particular sequence ofsteps, unless the context clearly indicates otherwise. Thus, in someembodiments, two or more of the steps of a method may be performedsimultaneously. Alternatively, in some other embodiments, two or more ofthe steps of a method may be performed sequentially. Further, in thefollowing specification, drawings, and in the claims, the embodimentsrelated to the methods are not limited to the order of appearance of thesteps in the claims, drawings, or in the specification. Thus, by way ofexample, in embodiments including steps (a), (b), and (c), the step (c)may be performed simultaneously with, prior to, or after the step (b).Further, in some embodiments, step (a) may be performed after step (b)or after step (c). In some other embodiments, all the three steps (a),(b), and (c) may be performed simultaneously.

As discussed in detail below, some embodiments of the invention aredirected to methods for treating a semiconductor material including achalcogenide material. The semiconductor material may be in the form ofa layer of semiconductor material that may be disposed on one or moreother layers. In some embodiments, the semiconductor material is acomponent of a photovoltaic device, such as a thin film photovoltaicdevice having multiple layers. In some embodiments, the semiconductormaterial serves as an absorber layer in the photovoltaic device. Theterm “absorber layer” as used herein refers to a semiconducting layerwherein the solar radiation is absorbed and generates electron-holepairs.

The term “chalcogenide” as used herein refers to a compound of at leastone chalcogen and at least one electropositive element. The term“chalcogen” refers to tellurium, selenium, or sulfur. Suitablechalcogenide materials include cadmium telluride, magnesium telluride,mercury telluride, lead telluride, zinc telluride, cadmium selenide,mercury selenide, lead selenide, zinc selenide, cadmium sulfide, mercurysulfide, zinc sulfide, lead sulfide, cadmium selenide telluride, cadmiumzinc telluride, cadmium sulfur telluride, cadmium manganese telluride,cadmium magnesium telluride, or combinations thereof. Theabove-mentioned semiconductor materials may be used alone or incombination. Further, these materials may be present in more than onelayer, each layer having different type of semiconductor material orhaving combinations of the materials in separate layers. In certainembodiments, the semiconductor material includes cadmium telluride(CdTe). In certain embodiments, the semiconductor material includesp-type cadmium telluride (CdTe).

Improved methods for processing such materials are presented in commonlyowned U.S. patent application Ser. No. 13/601,110, filed 31 Aug. 2012.In certain embodiments described therein, a semiconductor material istreated with a chemical agent comprising iodine. The iodine-containingchemical agent is used to react with the semiconductor material to forma chalcogen-rich region (such as a tellurium-rich region in CdTematerial) which promotes the formation of a stable, efficient ohmiccontact in photovoltaic devices. The chemical agent is formulated toprovide an effective quantity of elemental iodine in contact with thesemiconductor material to enable the formation of the chalcogen-richregion. However, maintaining consistent results when working with iodineformulations presents several challenges. Processes in which thesemiconductor material is immersed in a solution of dissolved iodine arepotentially subject to control issues due to contaminants and reactionbyproducts entering the solution, while iodine concentration tends tocontinually decrease due to volatilization and consumption. Applicationof aqueous iodine to a surface of semiconductor material such as CdTeresults in a change in the surface wetting properties of the CdTe, andthe aqueous iodine no longer adheres to the CdTe surface, preventingconsistent and uniform formation of the enriched tellurium region. Inaddition, the chalcogen-enrichment reaction often is performed atelevated temperatures. Applying thin films of aqueous iodine to a warmedCdTe surface, for example, results in volatilization of the iodine priorto completion of the tellurium-enrichment reaction.

Embodiments presented herein employ improved chemical agent compositionswith enhanced iodine stability (that is, lower susceptibility to iodinevolatilization) and/or enhanced iodine solubility. These embodimentsenjoy comparable performance to those employing aqueous immersiontechniques for applying the chemical agent, with added advantages ofimproved process consistency and ease of maintenance.

In one embodiment, a method for treating a semiconducting material isprovided, in which a semiconducting material comprising a chalcogenideis contacted with a chemical agent. In particular embodiments, thechemical agent is applied to the semiconducting material for the purposeof forming a chalcogen-rich region within the semiconducting material.For example, where the semiconducting material comprises tellurium, asin cadmium telluride materials, the chemical agent is applied to reactwith the telluride compound to form a tellurium-rich region.

The term “chalcogen-rich region” as used herein refers to a regionhaving an average atomic concentration of chalcogen greater than anaverage atomic concentration of the chalcogen in the bulk region (thatis, the region that is unaffected by contact with the chemical agent) ofthe semiconductor material. In some embodiments, the semiconductormaterial includes tellurium, and the application of the chemical agentis performed to form a tellurium-rich region in the semiconductormaterial. The term “tellurium-rich region” as used herein refers to aregion having an average atomic concentration of tellurium greater thana bulk region of the semiconductor material. In some embodiments, aratio of the average atomic concentration of tellurium in thetellurium-rich region to the average atomic concentration of telluriumin the bulk region of the semiconductor material is greater than about1.2. In some embodiments, a ratio of the average atomic concentration oftellurium in the tellurium-rich region to the average atomicconcentration of tellurium in the bulk region of the semiconductormaterial is greater than about 2.

In some embodiments, the tellurium-rich region may be furthercharacterized by an average atomic ratio of tellurium to cadmium. Insome embodiments, the tellurium-rich region has an average atomic ratioof tellurium to cadmium in a range greater than about 2. In someembodiments, the tellurium-rich region has an average atomic ratio oftellurium to cadmium in a range greater than about 10. In someembodiments, the tellurium-rich region has an average atomic ratio oftellurium to cadmium in a range greater than about 40. The term “atomicratio” as used herein refers to a ratio of average atomicconcentrations.

The chalcogen-rich region may be further characterized by its thickness.In some embodiments, the chalcogen-rich region has a thickness in arange greater than about 10 nanometers. In some embodiments, thechalcogen-rich region has a thickness in a range from about 10nanometers to about 1000 nanometers. In some embodiments, thechalcogen-rich region has a thickness in a range from about 50nanometers to about 500 nanometers. In some embodiments, the methods ofthe present invention may advantageously provide for a deeperchalcogen-rich region when compared to chalcogen-rich region formedusing conventional etching chemical agents.

The chemical agent applied in the present embodiments includes a solventand an iodine complex (“iodophor”) dissolved in the solvent. An iodophoris a complex formed between elemental iodine and an iodophor-formingsolubilizing agent. The chemical agent, in one embodiment, is theproduct of mixing the solvent, elemental iodine, and a suitableiodophor-forming solubilizing agent; upon mixing, the elemental iodinewill form the iodophor complex with the solubilizing agent. It should benoted that in some instances, the solvent itself forms an iodophor inthe presence of elemental iodine, and thus the solvent can be consideredboth the “solvent” component and the “solubilizing agent” mentionedabove; thus it may not be necessary in all instances to mix threedistinct components to form the chemical agent described herein. Withoutbeing bound by any particular theory, it is believed that the iodophorreduces the volatility of iodine in the agent and serves as a solublereservoir for elemental iodine, releasing a controlled amount ofelemental iodine into the agent as the iodine is removed during useand/or storage of the chemical agent. Experiments comparing the efficacyof chemical agents comprising soluble iodide compounds vs. agentscomprising dissolved elemental iodine suggest that elemental iodine isfar more effective than iodide in producing an effective chalcogen-richregion in the semiconducting materials of interest. Examples ofiodophors include polyvinyl pyrrolidone (also known as PVP, orpovidone):iodine, polyethoxy polypropoxyethanol:iodine, and the nonylphenyl ether of polyethylene glycol:iodine. Iodophors are also formedwhen iodine is dissolved in ethylene glycol, propylene glycol, glycerin,and some other organic solvents, and thus further examples of iodophorsmay include ethylene glycol:iodine, propylene glycol:iodine, andglycerin:iodine.

The solvent used in the chemical agent may be an organic solvent. Insome embodiments, the solvent further exhibits solubility in water andhas a boiling point greater than about 100 degrees Celsius, and inparticular embodiments, the boiling point is greater than about 150degrees Celsius. The solvent may include, for example, ethylene glycol,propylene glycol, glycerol, or combinations thereof. The use of suchorganic solvents may enhance wetting on semiconductor materials such asCdTe relative to aqueous solutions, may (as noted above) promote theformation of iodophors, and may have higher solubility for elementaliodine relative to water.

Elemental iodine may be added to form the chemical agent by introductionof any convenient source, such as solid elemental iodine, solutions ofiodine in aqueous potassium iodide solution, or by providing any otherconvenient source of elemental iodine. The iodine complexes with anysuitable present solubilizing agent to form the iodophor describedabove. Thus the “total iodine” content of the chemical agent is definedherein to be the sum of the amount of iodine that is present in theiodophor (“complexed iodine”) plus the amount of elemental iodinedissolved in the chemical agent but not present in a complex (“freeiodine”). Free iodine in the chemical agent may be present (1) in theamount dictated by chemical equilibrium with the iodophor in the agent,and, if applicable, (2) due to an excess of elemental iodine beyond whatis taken up by available solubilizing agent. In some embodiments, thetotal iodine in the chemical agent is present at a concentration of atleast about 0.1 g/L (0.01 weight %), and in certain embodiments theconcentration is at least about 1 g/L (0.1 weight %). For example, insome embodiments the concentration of total iodine in the chemical agentis in a range from about 0.01 weight % to about 1 weight %.

The chemical agent described herein may be applied in the form of aliquid or as a paste. In some embodiments, the chemical agent furtherincludes a thickener to increase the viscosity of the agent to providethe desired consistency, which may vary depending on the processingconditions. The type and amount of thickener added to the chemical agentreadily may be determined by the desired consistency for a givenprocess. In some embodiments, the thickener includes an inorganicmaterial, such as particles of aluminum oxide, fumed silica, or othersimilar materials. Organic materials may also be used as the thickener,either in addition to, or in place of, the inorganic thickener. Examplesof suitable organic thickeners include polymeric materials (such asderivatives of cellulose), polyvinyl pyrrolidone, and polyvinyl alcohol.

Whether applied as a paste or as a liquid, by immersion or byapplication of a layer on the surface of the semiconductor material, thecontacting of the semiconductor material with the chemical agent isperformed at a temperature and for a time suitable to form the desiredregion of chalcogen enrichment. Thus, in some embodiments, the methodincludes heating the semiconductor material to a temperature of at leastabout 45 degrees Celsius, and in particular embodiments the temperatureis at least about 65 degrees Celsius. The selection of time for theprocess may vary, depending in part on the temperature and desired levelof reaction. In some embodiments, the time is at least about 2 minutes,and in particular embodiments, at least about 5 minutes. In someembodiments, the time is up to about 20 minutes, and in particularembodiments, up to about 15 minutes. For example, in one embodiment, thetime is in a range from about 5 minutes to about 10 minutes. One exampleof a time and temperature combination is 5-10 minutes at a temperatureof about 65 degrees Celsius.

Upon reaching the desired time for contacting the semiconductingmaterial with the chemical agent, the chemical agent is then removedfrom contact with the semiconducting material. Removal may be done byany technique suitable to remove the chemical agent, and any byproductsof its reaction with the semiconductor material, from contact with thesurface of the semiconductor material. One example of such techniques isrinsing with a suitable medium such as water or other solvent.

The semiconductor material may be processed to incorporate one or moredopants before, during, and/or after contacting the semiconductormaterial with the chemical agent. The term “dopant” as used hereinrefers to a species added to the semiconductor material to alter one ormore properties, such as, for example, electrical properties. Examplesof such dopants include copper, silver, and gold. In one embodiment, adopant is introduced into the semiconductor material to form a firstregion in the semiconductor material, where the first region comprisesthe dopant. In some embodiments, a second region is formed in thesemiconductor material, where an average atomic concentration of thedopant in the second region is greater than an average atomicconcentration of the dopant in the first region. These steps may becarried out simultaneously with contacting the semiconducting materialwith the chemical agent, or in any convenient sequence.

In certain embodiments, the formation of the first and/or second regionsis accomplished by contacting the semiconductor material with acontacting composition that includes the desired dopant. The contactingcomposition may be in any form used in the art to introduce dopants intosemiconducting materials, including solids, liquid solutions orsuspensions, pastes, vapors, or combinations of these. In someembodiments, the contacting composition is a liquid solution of dopantin a solvent, such as a metal salt solution. In some embodiments, dopantmay be present in the contacting composition solution at a concentrationless than about 10 parts per million. In some embodiments, the dopant ispresent in the contacting composition solution at a concentration in arange from about 10 parts per billion to about 1000 parts per billion.In some embodiments, the dopant is present in the contacting compositionsolution at a concentration in a range from about 100 parts per billionto about 500 parts per billion. In particular embodiments, the dopantincludes copper.

The contacting composition solution described above used for forming thefirst and/or second regions may be applied to the semiconductor materialby any suitable method, such as by spraying or immersion. In someembodiments, the semiconductor material may be contacted with thecontacting composition solution at a temperature in the range from about25 degrees Celsius to about 100 degrees Celsius, and in particularembodiments, the temperature is in a range from about 60 degrees Celsiusto about 75 degrees Celsius. The time, in some embodiments, for whichcontact with the solution may be maintained is in the range from about 1minute to about 30 minutes, and in particular embodiments is in therange from about 2 minutes to about 10 minutes. After removing thesolution from the semiconductor material, the semiconductor material maybe further subjected to a heat treatment to assist in the incorporationof the dopant into the semiconductor material.

In some embodiments, an average atomic concentration of the dopant inthe second region formed in the techniques described above is greaterthan about 5×10¹⁸ atoms/cm³. In some embodiments, an average atomicconcentration of the dopant in the second region is in a range greaterthan about 1×10¹⁹ atoms/cm³. In some embodiments, an average atomicconcentration of the dopant in the second region is in a range fromabout 5×10¹⁸ atoms/cm³ to about 1×10²⁰ atoms/cm³.

As noted earlier, an average atomic concentration of the dopant in thesecond region is greater than an average atomic concentration of thedopant in the first region. In some embodiments, a ratio of the averageatomic concentration of the dopant in the second region to the averageatomic concentration of the dopant in the first region is greater thanabout 5. In some embodiments, a ratio of the average atomicconcentration of the dopant in the second region to the average atomicconcentration of the dopant in the first region is greater than about10. In some embodiments, a ratio of the average atomic concentration ofthe dopant in the second region to the average atomic concentration ofthe dopant in the first region is greater than about 50.

As noted above, the formation of first and/or second regions describedabove, that is, the introduction of dopants into the semiconductormaterial, may be performed in a step or series of steps that is separatefrom the application of the chemical agent. In one such embodiment, thechemical agent is substantially free of dopant, meaning that theconcentrations of dopants such as copper, silver, or gold in thechemical agent are below about 10 parts per billion. In particularembodiments, the chemical agent is substantially free of copper. In someembodiments the chemical agent is removed from contact with thesemiconductor material, and then a dopant is introduced into thesemiconductor material. The dopant may be introduced, for example, bycontacting at least a portion of the semiconductor material with acontacting composition comprising a dopant, such as copper, silver, orgold.

In other embodiments, the chemical agent and a source of dopant may beapplied simultaneously; in these embodiments, the chemical agent furthercomprises a dopant, such as by the addition of a metal compound that issoluble in the chemical agent. In such embodiments, then, the chemicalagent and the contacting composition may be merged or mixed together,with one composition performing the function of multiple agents. Thusduring the contacting step described previously, dopants such as copper,silver, or gold, for example, may be introduced simultaneously with theformation of the chalcogen-rich region. In some embodiments, dopant maybe present in the chemical agent at a concentration less than about 10parts per million. In some embodiments, the dopant is present in thechemical agent at a concentration in a range from about 10 parts perbillion to about 1000 parts per billion. In some embodiments, the dopantis present in the chemical agent at a concentration in a range fromabout 100 parts per billion to about 500 parts per billion. Inparticular embodiments, the dopant includes copper.

In some embodiments, the method includes contacting at least a portionof the semiconductor material with a passivating agent. As with thesteps described previously, this step may be performed sequentiallybefore or after any of the other described steps, or it may be combinedwith one or more steps and performed simultaneously with the step orsteps with which it was combined. The term “passivating agent” as usedherein refers to an agent capable of altering the physical orcompositional characteristics of the semiconductor layer resulting inimproved device performance. In some embodiments, the passivating agentmay allow for removing defect states along the grain boundaries. In someembodiments, for example, the passivating agent may allow for diffusionbetween the CdS and CdTe layers in CdS/CdTe-based photovoltaic devices,thus enabling an improved interface. In some embodiments, thepassivating agent includes cadmium chloride (CdCl2). In someembodiments, the method may include contacting at least a portion of thesemiconductor material with cadmium chloride or a cadmium chloridesource. In some embodiments, a portion of the semiconductor material maybe treated with a solution of CdCl2. In some embodiments, a portion ofthe semiconductor material may be treated with CdCl₂ vapor.

In some embodiments, the step of contacting at least a portion of thesemiconductor material with a passivating agent further includes a heattreatment. In some embodiments, the heat treatment step may be performedsubsequent to the step of contacting at least a portion of thesemiconductor material with the passivating agent. In some embodiments,the heat treatment step may be performed simultaneously with the step ofcontacting at least a portion of the semiconductor material with thepassivating agent.

In some embodiments, the heat treatment step is performed at atemperature within a range from about 300° C. to about 500° C. In someembodiments, the heat treatment step is performed at a temperaturewithin a range from about 350° C. to about 450° C. In some embodiments,the heat treatment step is performed for a time duration within a rangefrom about 1 minute to about 60 minutes. In some embodiments, the heattreatment step is performed for a time duration within a range fromabout 10 minutes to about 45 minutes. In some embodiments, the heattreatment step is performed in an inert environment. In some otherembodiments, the heat treatment step is performed in an environmentincluding an oxidizing environment. Non-limiting examples of oxidizingenvironments include air or oxygen.

In some embodiments, the method may further include contacting at leasta portion of the semiconductor material with a cleaning agent, after thestep of treating the semiconductor material with the passivating agentto remove any impurities, such as, for example, cadmium oxide from thesurface. Suitable non-limiting examples of a cleaning agent include anaqueous solution of ethylene diamine (EDA), ammonium hydroxide (NH₄OH),or combinations thereof.

Some embodiments of the present invention include combinations of thevarious steps described above. For example, with reference to theFIGURE, one embodiment is a method for treating a semiconductor material110, wherein the semiconductor material 110 comprises a chalcogenide.The method includes (a) contacting at least a portion of thesemiconductor material 110 with a passivating agent; (b) forming a firstregion 112 in the semiconductor material by introducing a dopant intothe semiconductor material; (c) forming a chalcogen-rich region; and (d)forming a second region 114 in the semiconductor layer, the secondregion comprising a dopant, wherein an average atomic concentration ofthe dopant in the second region is greater than an average atomicconcentration of the dopant in the first region. Step (c) includescontacting at least a portion of the semiconductor material with thechemical agent described previously herein, wherein the chemical agentcomprises a solvent, elemental iodine, and an iodophor dissolved in thesolvent.

In any of the embodiments described herein, the step of forming a secondregion 114 may further include forming a chalcogenide of the dopantspecies. In some embodiments, the step of forming a second region 114may further include forming a telluride of the dopant species. Incertain embodiments, the second region 114 may include copper telluride.In some embodiments, the chalcogen-rich region (not shown) includes thesecond region 114. In some other embodiments, the second region 114includes the chalcogen-rich region. In some other embodiments, thechalcogen-rich region and the second region 114 are substantiallyoverlapping. In some embodiments, such as embodiments wheresemiconductor material 110 is included as a layer in a photovoltaicdevice, first region 112 may function primarily as the absorber in thedevice, while second region 114 may function to promote ohmic contactfor conducting current out of the device. It will be appreciated thatother layers and materials, such as graphite and/or metallic materials,may be disposed over second region 114 to complete the formation of anelectrode.

In some embodiments, a method for making a photovoltaic device ispresented. The method includes disposing a chalcogen containingsemiconductor material on a support, and processing the semiconductormaterial in accordance with any of the methods described above. In someembodiments, the semiconductor material is an absorber materialcomprising a chalcogenide as previously described. Methods forfabrication of thin-film photovoltaic devices in substrate orsuperstrate configurations are well known in the art. Examples can befound, for instance, in the aforementioned U.S. patent application Ser.No. 13/601,110.

EXAMPLES Example 1

A chemical agent paste was prepared by mixing 1 gram of fumed silica(0.2-0.3 micrometers diameter), 5 grams of propylene glycol, and 1 dropof a solution containing 1.0 molar elemental iodine in 1.0 molarpotassium iodide. The paste was applied by brush coating onto cadmiumtelluride samples. The coated samples were heated on a hot plate for 2minutes, and then the paste was removed with a water rinse. The pastedid not lose its ability to wet the surface of the samples as theexposure occurred, in contrast to behavior observed for aqueouscompositions. The paste remained orange colored, indicating that iodinewas present throughout the exposure. A tellurium-enriched region wasobserved to form on the exposed area of the samples.

Example 2

Thin film photovoltaic devices were fabricated using cadmium telluride(CdTe) as the absorber layer. The CdTe layer of the devices, afterdeposition, were treated with a paste chemical agent made by mixing 8grams of fumed silica, 50 grams of propylene glycol, and 0.5 ml ofiodine solution (1.0 moles/liter potassium iodide plus 0.5 moles/literelemental iodine). Various devices were respectively subjected to one ofseveral time-temperature combinations, with times ranging from 2 minutesto 20 minutes and temperatures ranging from 65 degrees Celsius to 150degrees Celsius. A control group was treated using an immersion processin an aqueous iodine solution. After treatment with theiodine-containing chemical agent, fabrication of the devices wascompleted and then the resultant devices were tested in highlyaccelerated life tests (HALT). Several of the experimentally processeddevices exhibited comparable performance to that of the control devices.Devices made with shorter exposure times and lower temperatures tendedto have decreased open circuit voltage during HALT. Devices made withlonger exposure times tended to show decreased open circuit resistanceduring HALT. In this particular example, the devices with best overallbehavior were those treated for 5 minutes-10 minutes at 65 degreesCelsius.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

The invention claimed is:
 1. A method for treating a semiconductormaterial, comprising: reacting a semiconductor material with an iodinecontaining chemical agent, wherein the iodine containing chemical agentcomprises a complex between a solvent and elemental iodine.
 2. Themethod of claim 1, wherein the semiconductor material comprises achalcogenide.
 3. The method of claim 2, further comprising a step offorming a chalcogen-rich region within the semiconductor material. 4.The method of claim 3, wherein the chalcogen-rich region is atellurium-rich region.
 5. The method of claim 1, further comprisingcontacting at least a portion of the semiconductor material with acontacting composition comprising a dopant to incorporate the dopantinto the semiconductor material.
 6. The method of claim 5, wherein thestep of contacting at least a portion of the semiconductor material witha contacting composition further comprises forming a chalcogenidespecies of the dopant.
 7. A method for making a photovoltaic device,comprising disposing a chalcogen containing semiconductor material on asupport; contacting at least a portion of the semiconductor materialwith a passivating agent; introducing a first dopant to thesemiconductor material and forming a first region in the semiconductormaterial; reacting the semiconductor material with an iodine containingchemical agent to form a chalcogen rich region; introducing a seconddopant to the semiconductor material and forming a chalcogenide speciesof the second dopant.
 8. The method of claim 7, wherein the first dopantand second dopant are the same.
 9. The method of claim 7, wherein thefirst dopant and second dopant are different.
 10. The method of claim 7,wherein the passivating agent is one of a solution of cadmium chlorideand a cadmium chloride vapor.
 11. The method of claim 7, wherein thestep of contacting at least a portion of the semiconductor material witha passivating agent further includes a heat treatment.
 12. The methodclaim of 11, wherein the heat treatment is performed at a temperaturewithin a range from about 300° C. to about 500° C.
 13. The method ofclaim 11, wherein the heat treatment step is performed for a timeduration within a range from about 1 minute to about 60 minutes.
 14. Themethod of claim 11, wherein the heat treatment is performed in an inertenvironment.
 15. The method of claim 11, wherein the heat treatment isperformed in an oxidizing environment.
 16. The method of claim 7,further comprising the step of contacting at least a portion of thesemiconductor material with a cleaning agent, after the step ofcontacting at least a portion of the semiconductor material with apassivating agent.
 17. The method of claim 16, wherein the cleaningagent is one of ethylene diamine, ammonium hydroxide, and a combinationof ethylene diamine and ammonium hydroxide.